Method for the production of biodegradable foamed products

ABSTRACT

A method for the production of a biodegradable foamed product  7  from a base material and a blowing agent  1.  The base materials are mixed with a blowing agent and any other required additives  1.  The mixture  3,  after extrusion  2,  is placed in a microwave transparent mold  6  and processed in a microwave via distinct steps. The first step preheats the extrudate to a temperature just below the flash point  6.  The second step  7  rapidly heats the extrudate beyond the flash point causing the extrudate to foam in the mold  7.  By utilizing this method it is possible to produce shaped articles with uniform properties and with packaging properties such as compressibility, resilience and shock absorption.

[0001] The present invention relates to the method for manufacturingbiodegradable moulded products. More particularly, the present inventionrelates to having a matrix based on starch, or materials with similarTheological properties, foamed for use in moulded products. Inparticular, the method involves two or more distinctive steps duringmicrowave processing, giving a product with improved packagingproperties including resilience, compressibility and shock absorption.

BACKGROUND ART

[0002] The area of starch based biodegradable foamed materials is widelydiscussed in the prior art. In particular, U.S. Pat. No. 6,168,857 has adetailed discussion that may be referenced in relation to this patent.

CONTAINERS AND PACKAGING

[0003] Articles such as sheets, films and packaging moulds made frommaterials such as paper, paperboard, plastic, polystyrene, and evenmetals are used in enormous quantity. This can take the form of printedmaterials, mats, containers, separators, dividers, envelopes, lids,tops, cans, and other packaging materials. Advanced processing andpackaging techniques presently allow an enormous variety of liquid andsolid goods to be stored, packaged, or shipped while being protectedfrom harmful elements.

[0004] Containers and other packaging materials protect goods fromenvironmental influences and distribution damage, particularly fromchemical and physical influences. Packaging helps protect an enormousvariety of goods from gases, moisture, light, micro-organisms, vermin,physical shock, crushing forces, vibration, leaking, or spilling.

[0005] For the purposes of the discussion, many prior art products andprocesses are seen as not being environmentally friendly. Wherein forthe purposes of the present invention, “environmentally friendly” may becharacterised as:

[0006] Being produced from substantially naturally occurring andrenewable, raw materials;

[0007] Manufactured in such a way as to cause minimal deterioration tothe environment for example via low energy processing and low emissionmethods;

[0008] Producing a product that is biodegradable and not harmful to theenvironment; and

[0009] Production whereby the whole process is sustainable.

[0010] However it is not intended that this definition be seen aslimiting.

THE IMPACT OF TRADITIONAL MATERIALS

[0011] Recently there has been a debate as to which of these materials(e.g., paper, paperboard, plastic, polystyrene, glass, or metal) is mostdamaging to the environment. Paper, paperboard, plastic, polystyrene,glass, and metal materials each have unique environmental issues that donot meet the definition of “environmentally friendly”. These issues canrelate to the biodegradability of the material itself or the method ofproduction, for example, high energy use, damaging by-products andemissions.

[0012] Another problem with paper, paperboard, polystyrene, and plasticis that each of these requires relatively expensive organic startingmaterials, some of which are non-renewable, such as the use of petroleumin the manufacture of polystyrene and plastic. Although trees used inmaking paper and paperboard are renewable in the strict sense of theword, their large land requirements and rapid depletion in certain areasof the world undermines this notion. Hence, the use of huge amounts ofessentially non-renewable starting materials in making sheets andarticles therefrom cannot be sustained and is not wise from a long-termperspective.

STARCH BASED FOAMS

[0013] Recent uses of starches and starch derivatives as the bindingagent or sole constituent within moulded articles are known. U.S. Pat.No. 5,095,054 is the parent document for this style of product. Theparent patent, and the patents citing this patent, recognise the factthat starch can be foamed and moulded by means of forming what is knownin the art as “destructurised starch”. In the manufacture ofdestructurised starch, native starch or starch derivatives are mixedwith a wide variety of additives such as plasticisers, and heated,solidified and cooled, typically into a mould.

[0014] EP-707034 and WO95/07693 both use conventional thermal conductiveheating processes that do not lend themselves to the production ofthick-walled mouldings. Non-homogenous heating occurs when the heatingprocess is reliant on heat conduction as it is difficult to heat thecore of the material to the same extent as the exterior. This results innon-uniform foam properties, which is undesirable in protectivepackaging used for cushioning applications.

[0015] A further example includes U.S. Pat. No. 6,168,857 in which theprocess is only usable in thin walled applications. Thin walled articlesare of limited use in terms of protective packaging used for cushioningapplications. Thick walled articles are needed where shock absorptionproperties are required. The method of fashioning articles from sheetsused in U.S. Pat. No. 6,168,857 does not allow for the forming of thicksheets.

[0016] Another patent U.S. Pat. No. 5,730,824 utilises extrusion toproduce foam panels. These panels are then laminated together to formthick sheets, which can be wire cut to varying size shapes. There arelimitations in this process due to the expensive capital equipmentrequired for manufacturing. As a result of the expensive equipment, themethod necessitates shipping ‘air’ as the product can only be made incentral locations. In addition the shapes are either very limited orcostly because they have to be cut out of sheets instead of mouldedduring the foaming process.

[0017] Another example, U.S. Pat. No. 5,801,207, relates to takingfoamed starch pieces, placing them in a bag or within layers of sheetingand moulding the pre-expanded peanuts into solid foam-in-place moulds.The limitations are that the foamed peanuts used to make the moulds arevery bulky and take up a lot of store space, and again increase expensethrough having to ship air to the point of use instead of sending densepellets that can be foamed at point of use. The method is also acomplicated procedure for the end-user, as they have to fill and sealbags of foamed peanuts and then mould the bag to the product shape.

[0018] Two further patents, WO 9,851,466 and U.S. Pat. No. 5,639,518,utilise dielectric heating in processing the starch based materials.

[0019] In WO 9,851,466, the dielectric heating proceeds in one step anddoes not take into account the changing dielectric properties of thematerial as it heats, nor the relationship between the rheologicalproperties (for example elasticity and viscosity) and the rate ofheating. This results in the material not being heated as rapidly andintensely, thus lowering the potential foaming and product resilience.

[0020] U.S. Pat. No. 5,639,518 again does not utilise different stepsduring processing to take account of the changing dielectric rheologicalproperties of the material as it heats up. Two stages are outlinedrelating to changes in the microwave frequency from low frequency andthen high frequency but not with any reference to varying materialproperties and a rate of heating profile. This frequency change resultsin a significant increase in processing expense due to more specialisedequipment being required.

[0021] In addition, the methods described above often produce foams withvarying consistency depending on the shape required and often withoutthe combination of uniform physical and mechanical properties. Theseproperties include density, compressibility, resilience and shockabsorption. All of these properties limit the product applications.

[0022] A further method for moulding starch-based mixtures into articlesinvolves batch-moulding an aqueous starch mixture between heated dies.The starch binder is preferably initially in an unmodified,un-gelatinised state within the mouldable aqueous mixture. Thestarch/water mixtures are heated between the moulds to a temperaturegreat enough to gelatinise the starch as well as to remove the majorityof the water from the mouldable mixture. The resulting moulded articlescan be de-moulded, but are initially very brittle until they have been“conditioned” by placing them in a high humidity chamber for extendedperiods of time in order to reabsorb moisture. While the foregoing batchmoulding process may have some utility, it does not allow for continuousmanufacturing and as such is expensive to run.

[0023] Based on the above, there is a perceived difficulty in findingimproved methods for manufacturing low cost and environmentally friendlyproducts, which have properties similar to paper, paperboard,polystyrene, or plastic, but are biodegradable and resilient.

[0024] An object of the present invention is the provision of the methodto produce a foamed product with uniform physical and mechanicalproperties such as density, compressibility, resilience and shockabsorption.

[0025] A further object is the provision of a method and product, whichovercome some, or all of the above described disadvantages of existingbiodegradable foamed products.

[0026] Another object is the production of a biodegradable foamedproduct and method, which provides the public with a useful alternativeto existing methods and products.

DISCLOSURE OF INVENTION

[0027] According to one aspect of the present invention there isprovided a method of producing a biodegradable foamed product, themethod including the steps of;

[0028] (a) selecting one or more base materials,

[0029] (b) blending the base materials with one or more additives toform a mixture;

[0030] (c) extruding the mixture in the presence of water;

[0031] (d) transferring the extrudate to a mould;

[0032] (e) heating the extrudate using dielectric heating, characterisedby the steps of;

[0033] (f) preheating the extrudate to below the flash point of theblowing agent to achieve a uniform temperature distribution throughoutthe extrudate;

[0034] (g) rapid heating of the extrudate through the flash point of theblowing agent thereby forming an expanded foamed product.

[0035] It is the understanding of the applicant that the advantages ofthis method of introducing energy are based on the fact that withdielectric heating it is possible to realise very high power densities.In addition, the energy acts on the starch material not merely at thesurface, but also penetrates into the starch material, allowing rapidinternal heating of the material, which results in the flashing off ofthe blowing agent. Uniform cell structure is achieved through theoptimisation of microwave application such that a uniform electric fieldis generated within the material.

[0036] It is understood by the applicant that the bubble growth is afunction of the amount of energy delivered to the blowing agent and ofthe rheological properties of the material. The viscoelasticity of thematerial must be such that it can allow the build up of water vapourpressure without rupture or severe shrinkage.

[0037] In the preferred embodiment, dielectric heating is used forheating the product as it has been found in practice to provide the mosteven heating and also the required intensity.

[0038] According to a further aspect of the present invention there isprovided a method of producing a biodegradable foamed product, themethod including the steps of;

[0039] (a) selecting one or more base materials,

[0040] (b) blending the base materials with one or more additives toform a mixture;

[0041] (c) extruding the mixture in the presence of water;

[0042] (d) transferring the extrudate to a mould;

[0043] (e) heating the extrudate using dielectric heating, characterisedby the steps of;

[0044] (f) preheating the extrudate to below the flash point of theblowing agent to achieve a uniform temperature distribution throughoutthe extrudate;

[0045] (g) rapid heating of the extrudate through the flash point of theblowing agent thereby forming an expanded foamed product; wherein, theforce of the water vapour pressure is higher than the viscous forces ofthe material.

[0046] If the viscosity of the extrudate melt is too low, no watervapour pressure will build up as the material will offer no resistanceagainst which the water vapour pressure can build.

[0047] According to a further aspect of the present invention there isprovided a method of producing a biodegradable foamed product, themethod including the steps of;

[0048] (a) selecting one or more base materials,

[0049] (b) blending the base materials with one or more additives toform a mixture;

[0050] (c) extruding the mixture in the presence of water;

[0051] (d) transferring the extrudate to a mould;

[0052] (e) heating the extrudate using dielectric heating, characterisedby the steps of;

[0053] (f) preheating the extrudate to below the flash point of theblowing agent to achieve a uniform temperature distribution throughoutthe extrudate;

[0054] (g) rapid heating of the extrudate through the flash point of theblowing agent thereby forming an expanded foam product; wherein, theelasticity of the material is such that it is high enough to prevent theblowing agent vapour rupturing the extrudate and low enough to preventsevere shrinking.

[0055] If the elasticity is too low during step (g), the water vapourpressure will rupture the extrudate causing a weak open cell structureto be formed. Alternatively, if the elasticity at step (g) is too high,the material will shrink before the structure is able to solidifycausing a very high density and hard foam to be formed.

[0056] It is the understanding of the applicant that viscosity andelasticity are both functions of a large number of variables includingtemperature, water content, molecular structure (influenced by the levelof molecular degradation), plasticiser content, additive content forexample viscosity modifiers and starch gelatinisation.

[0057] In tests completed on the material processed using the abovemethod, it has been found that the level of molecular degradationaffects the foam success as shown on Graph 1 below:

[0058] Graph 1 shows that the foam success is influenced by the level ofmolecular degradation, (as indicated by water solubility index, WSI).Those skilled in the art will appreciate that a correlation thereforeexists between viscosity and elasticity of the base material and thelevel of molecular degradation.

[0059] According to a further aspect of the present invention, themicrowave equipment is adjusted such that maximum heating (incorporatingpower density) is rapidly delivered to the material in stage (g). In afurther embodiment, in stage (g) described above, one or more re-tuningsteps may be effected depending on the dielectric property changes ofthe extrudate. Alternatively, separate microwave generators, differentlytuned, are used in place of the re-tuning steps.

[0060] Varying the intensity of thermal energy as shown in Graph 2 belowregulates the foaming success:

[0061] In the case of the present invention, it is understood by theapplicant that the foam formation involves the nucleation and growth ofwater vapour bubbles as the result of rapid heating.

[0062] The applicant has found that the more rapid the heating is, thebetter the foam success

, however it will be appreciated by a person skilled in the art thatthere is an upper limit after which foam success deteriorates. Foamsuccess is measured based on the characteristics of successful packagingfoams. These include density, compressibility, resiliency and particleadhesion but it is the combination of these properties that ultimatelydetermines the functionality of the foam. The quantity ‘foam success’ isused in the application to quantify the overall functionality of thefoam.

[0063] Hence, given the relationship between viscosity, elasticity andrapid heating, in the preferred embodiment, between step (f) and step(g), the microwave equipment is re-tuned.

[0064] In addition to the changes in the viscosity and elasticity, thedielectric properties of the extrudate change considerably between roomtemperature and around 80-99° C. (just below the flash point). It hasbeen found that by re-tuning the microwave equipment, it is re-matchedto the different dielectric properties of the material at the end of thepreheat phase.

[0065] With the equipment/material matching improved, higher powerdensities can be achieved and the rate of heating can be optimised inthe critical heating region i.e. around 90-130° C. where the foamingoccurs i.e. in step (g). More accurate power density profiles can alsobe achieved through re-tuning.

[0066] For the purposes of the explanation two stages (f) and (g) havebeen used for an example of the dielectric heating method. This shouldnot be seen as limiting, as further stages are possible before and afterreaching the material flash point.

[0067] In the preferred embodiment, the microwave radiation is withinthe standard microwave frequency range 915 MHz to 25 GHz. Preferably,the radiation is in the range easily reached for microwave equipment of2.0 GHz to 3.0 GHz, more preferably in the standard range for microwaveequipment of 2.40 to 2.50 GHz. According to a further aspect of thepresent invention, the operating frequency of the microwave is heldapproximately constant during processing. In addition, a power densitydistribution in the range of 0.01 W/cc to 10.00 W/cc is used. Morepreferably the power density is greater than 3.5W/cc.

[0068] In a further embodiment, during microwave treatment, the poweroutput of the microwave is varied depending on the desired productcharacteristics required. Varying the duty cycle of the microwave sourceis a commonly used means of controlling the effective power output.Alternatively, if more than one microwave generator is used, they may beat different power ratings.

[0069] In the above described method, the base material at step (a) isselected from the group consisting of: proteins, starches includingcereal, root and tuber starches, modified starches, food residues,biodegradable polymers, and any combination thereof. It can also includematerials where the material has rheological properties that are similarto those of starch materials.

[0070] In the preferred embodiment, the preferred blowing agent iswater. It is understood by the applicant that successful foaming comesfrom rapid heating, which causes a ‘flash-off’ of the agent. In the caseof water, if the rate of heating is too slow, then the rate of vapourtransfer will be higher than rate of heating. This means the water willevaporate, causing no build up in water vapour pressure and hence nowater vapour bubbles form or grow to create the ‘cells’ necessary for auniform foamed product.

[0071] The present invention also provides a method as described abovein which the further additives include a blowing agent other than water.

[0072] In a preferred embodiment, a solid blowing agent is added at step(b). Alternatively a liquid blowing agent is added at either step (b) orstep (c). The process is not limited to only occur as the result of thechange of state of the blowing agent from a liquid into the gaseousphase. There are other mechanisms for foam formation including thethermal degradation of solids to form gases e.g. sublimation orthermally initiated chemical reactions, which produce gases. In apreferred embodiment these gases act as blowing agents.

[0073] Other additives can also be included. Typically these additivesare selected from a range of biodegradable plasticisers (such aspolyvinyl alcohol with various hydrolysis degrees), nucleating agents(such as magnesium silicate, calcium carbonate), processing aids (suchas lecithin, mono-glycerides) and any combination thereof.

[0074] Further additives with an application dependent function can alsobe included such as flame retardants, fungus and mould inhibitors,strength adjusting additives, adhesion promoters, viscosity modifiers,fillers and rodent repellents.

[0075] According to a further aspect of the present invention, in theabove described method, the moisture content, during extrusion, rangesbetween 10% by weight and 50% by weight. Preferably the range is from15% to 30% by weight as this has been found to give the best foamingsuccess in trials. These weight ranges are based upon the total weightof the raw material starting mixture before the microwave heatingthereof.

[0076] A further option for processing includes a conditioning stepfollowing extrusion. The extrudate is conditioned to a pre-determinedmoisture content via temperature and humidity control. The moisturecontent is in the range from 5% to 20% by weight as this has been foundto give the best foaming success in trials.

[0077] During steps (f) and (g), the extrudate is heated dielectricallyusing moulds to hold the extrudate. The mould container is eithercompletely microwave transparent or at least largely microwavetransparent, taking into account the increase in volume of the material,and the fact that the water vapour and air needs to be expelled from themould unhindered at the same time.

[0078] A further advantage of the present invention is the low set-upand running cost. The method utilises equipment generally well known andutilises, for example extruders and microwave ovens. Similarly, thematerials can be shipped prior to foaming, thus keeping freight costslower due to the smaller volumes shipped. The customer can thenmicrowave and foam the material thus improving the process efficiencyi.e. not paying to ship ‘air’ in the expanded product and spreading thecapital equipment cost.

[0079] According to another aspect of the present invention, there isprovided a biodegradable foamed product produced by any one of the abovedescribed methods. According to another aspect of the present invention,there is provided a product produced after step (c).

[0080] According to another aspect of the present invention, there isprovided a biodegradable product produced after step (f).

BRIEF DESCRIPTION OF DRAWINGS

[0081] Further aspects of the present invention will become apparentfrom the following description, which is given by way of example onlyand with reference to the accompanying drawings in which:

[0082]FIG. 1 is a flow diagram of the process according to a preferredembodiment of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION EXAMPLE 1

[0083] Referring to the drawing, the method starts with a blend of astarch (as the base material) and other substances mixed together toform a blend (1).

[0084] An example of a base material blend termed ‘A2’ for the example,includes: Proportion Material [% Dry weight] Tapioca Starch 81.25Maltodextrin (dextrose equivalent 8-12) 2.5 Pregelatinsed modified waxymaize starch 3.00 Polyvinyl alcohol (degree of hydrolysis = 86-89 mole%) 12.00 Magnesium Silicate 0.25 Lecithin 1.00

[0085] However, it will be appreciated that other biodegradable polymerscan be used as the base material, provided the rheological properties ofthe material is similar to those of starch pellets.

[0086] The blend (1) is extruded in an extruder (2) of known type at amoisture content of 15% to 30% by weight to achieve an extrudate withthe desired Theological properties (3). Such an extruder is, forexample, one with a single screw.

[0087] The extrudate (3) is cut into pellets or long rods or rolled intofinal mould ready shapes. The extrudate is conditioned (4) to a moisturecontent in the range of 12% to 16% by weight at a temperature of 15° C.to 40° C. This occurs in 25% to 80% relative humidity in a controlledclimate container (4) to minimise case hardening of the extrudate (3).This gives a shelf stable and easily transported product, which can bestored for later use (5).

[0088] It will be appreciated by those skilled in the art that othershapes may also be achieved, for example plain or patterned sheets.

[0089] The conditioned extrudate (5) is loaded into amicrowave-transparent mould in the shape required. For this example, 70g of extrudate (5) is loaded into an 18 cm×8 cm×4 cm container (6).

[0090] The mould is placed into a microwave field (6) using 2.45 GHzfrequency microwave energy with a variable power output from 100 W to 5kW, for the example set to a 2 kW source. The duty cycle of themicrowave is 100%.

[0091] For stage A, of the preferred embodiment, the extrudate (5) ispre-heated for less than 30 seconds depending on the tuning of themicrowave generator to bring the extrudate (5) to just below the flashpoint of the water blowing agent. The magnetron (not shown) is adjustedto a power density of 0.83 W/cc whence it is used to heat the extrudate(5) in the mould (6) for less than 30 seconds depending on the tuning ofthe microwave equipment (not shown). This produces a uniform, resilient,moulded foam block (7).

[0092] Re-tuning the magnetron if necessary allows for changingdielectric properties in the extrudate as the temperature changes.Better power densities and more accurate density profiles can thus beachieved. While re-tuning is carried out the frequency remainsapproximately around 2.45 GHz. A temperature profile is shown for theexample in Graph 3 below.

[0093] The result of heating at 100% duty cycle is foam success of 8,shown in Graph 2 above. Foam success is measured based on thecharacteristics of successful packaging foams. These include density,compressibility, resiliency and particle adhesion but it is thecombination of these properties that ultimately determines thefunctionality of the foam.

EXAMPLE 2

[0094] In a second example the same base material, A2 is put through thesame steps (1) to (5) described above. During steps (6) and (7) themicrowave duty cycle is changed from 100% in example 1, to 70%.Accordingly, the length of time for each step is also varied to accountfor the altered heating profile. Graph 3 shows the resulting heatingprofile and Graph 2 shows the resulting foam success is 7.

EXAMPLE 3

[0095] In a third example the same base material, A2 is put through thesame steps as example 2 above however a microwave duty cycle 50% isused. The length of time for each step is again varied to account forthe altered heating profile. Graph 3 shows the resulting heating profileand Graph 2 shows the resulting foam success is 6.

EXAMPLE 4

[0096] In a fourth example the same base material, A2 is put through thesame steps (1) to (5) described in example 1. During steps (6) and (7)the microwave duty cycle is held at 100% and the foaming success istested with a 60 second single step process with the magnetron tuned tothe material at the beginning of the microwave process. The foam successresult of 5 is shown in Graph 4 below.

EXAMPLE 5

[0097] In a fifth example the same base material as example 1 isprocessed as per example 4 above, except that during steps (6) and (7)the foaming success is tested. This is done with a 60 second single stepprocess wherein the magnetron is tuned to the material at the normaltuning used when moving through the flash temperature of the blowingagent. The foam success result of 2 is shown in Graph 4 above.

EXAMPLE 6

[0098] In a sixth example the same base material as example 1 isprocessed as per example 4 above except that during steps (6) and (7)the foaming success is tested with two 30 second steps wherein themagnetron re-tuned to the material at the end of each step. The foamsuccess result of 8 is shown in Graph 4 above.

[0099] Aspects of the present invention have been described by way ofexample only and it should be appreciated that modifications andadditions may be made thereto without departing from the scope thereofas defined in the appended claims.

The claims defining the invention are:
 1. According to one aspect of thepresent invention there is provided a method of producing abiodegradable foamed product, the method including the steps of; (a)selecting one or more base materials, (b) blending the base materialswith one or more additives to form a mixture; (c) extruding the mixturein the presence of water; (d) transferring the extrudate to a mould; (e)heating the extrudate using dielectric heating, characterised by thesteps of; (f) preheating the extrudate to below the flash point of theblowing agent to achieve a uniform temperature distribution throughoutthe extrudate; (g) rapid heating of the extrudate through the flashpoint of the blowing agent thereby forming an expanded foamed product.2. A method of producing a biodegradable foamed product, the methodincluding the steps of; (a) selecting one or more base materials, (b)blending the base materials with one or more additives to form amixture; (c) extruding the mixture in the presence of water; (d)transferring the extrudate to a mould; (e) heating the extrudate usingdielectric heating, characterised by the steps of; (f) preheating theextrudate to below the flash point of the blowing agent to achieve auniform temperature distribution throughout the extrudate; (g) rapidheating of the extrudate through the flash point of the blowing agentthereby forming an expanded foamed product, wherein the force of thewater vapour pressure is higher than the viscous forces of the material.3. A method of producing a biodegradable foamed product, the methodincluding the steps of; (a) selecting one or more base materials, (b)blending the base materials with one or more additives to form amixture; (c) extruding the mixture in the presence of water; (d)transferring the extrudate to a mould; (e) heating the extrudate usingdielectric heating, characterised by the steps of; (f) preheating theextrudate to below the flash point of the blowing agent to achieve auniform temperature distribution throughout the extrudate; (g) rapidheating of the extrudate through the flash point of the blowing agentthereby forming an expanded foamed product; wherein, the elasticity ofthe material is such that it is high enough to prevent the blowing agentvapour rupturing the extrudate and low enough to prevent severeshrinkage.
 4. A method as claimed in any one of claims 1 to 3, whereinthe initial water content is in the range from 10% and 50% by weight. 5.A method as claimed in any one of claims 1 to 4, wherein the extrudateis conditioned to a pre-determined moisture content in the range of 5%to 20% by weight prior to step (d).
 6. A method as claimed in any one ofclaims 1 to 5 wherein the mould containers used in microwave treatmentare either completely microwave transparent or at least largelymicrowave transparent.
 7. A method as claimed in any one of claims 1 to6 wherein the microwave frequency is in the range from 915 MHz to 25GHz.
 8. A method as claimed in any one of claims 1 to 6 wherein themicrowave frequency is in the range from 2 GHz to 3 GHz.
 9. A method asclaimed in any one of claims 1 to 6 wherein the microwave frequency isin the range from 2.40 GHz to 2.50 GHz.
 10. A method as claimed in anyone of claims 1 to 6 wherein a single frequency is used for microwavetreatment.
 11. A method as claimed in any one of claims 1 to 10 whereinthe microwave power density is in the range from 0.01 W/cc to 10.00W/cc.
 12. A method as claimed in any one of claims 1 to 11 wherein themicrowave equipment is tuned to take into account variations in thechanging dielectric properties of the material.
 13. A method as claimedin any one of claims 1 to 12 wherein the microwave equipment utilisesseparate microwave generators, to take into account variations in thechanging dielectric properties of the material.
 14. A method as claimedin claim 13 wherein the said microwave generators are tuned to afrequency selected from the group consisting of; all generators at thesame frequency; all generators at different frequencies; andcombinations thereof.
 15. A method as claimed in any one of claims 1 to14 wherein the microwave output is adjusted at any stage duringmicrowave processing.
 16. A method as claimed in any one of claims 1 to15 wherein a plurality of microwave generators is are used and whereinthe output of the said generators is selected from the group consistingof; the same output for all generators; different outputs for allgenerators; or combinations thereof.
 17. A method as claimed in any oneof claims 1 to 16 wherein the extrudate is heated in two or more stepsduring the foaming process.
 18. A method as claimed in any one of claims1 to 17 wherein the base material at step (a) is selected from the groupconsisting of: proteins, starches including cereal, root and tuberstarches, modified starches, food residues, biodegradable polymers, andany combination thereof.
 19. A method as claimed in any one of claims 1to 18 wherein the base material at step (a) is made up of materials withsimilar rheological properties to claim
 17. 20. A method as claimed inany one of claims 1 to 19 wherein a base material is used at step (d)with rheological properties that match the extruded material.
 21. Amethod as claimed in any one of claims 1 to 20 wherein the primaryblowing agent is water.
 22. A method as claimed in any one of claims 1to 21 wherein the primary blowing agent is a solid material that isadded at step (b) to form a mixture.
 23. A method as claimed in any oneof claims 1 to 22 wherein the primary blowing agent is a liquid materialthat is added at either step (b) or step (c).
 24. A method as claimed inany one of claims 1 to 23 wherein additives from any of the followingare included, being selected from plasticisers, nucleating agents,strength adjusting agents, viscosity modifiers, adhesion promoters,processing aids and fillers.
 25. A method as claimed in claim 24 whereinthe plasticiser is polyvinyl alcohol.
 26. A method as claimed in claim24 wherein the nucleating agent is magnesium silicate.
 27. A method asclaimed in any one of claims 1 to 26 wherein additives are included,said additives included to aid dielectric properties, blowing agents,flame retardant properties, anti-fungal agents, mould inhibitors, andany combination of two or more of these.
 28. A biodegradable foamedproduct produced by the method as claimed in any one of claims 1 to 27.29. An intermediate product resulting from steps (c) of the method asclaimed in any one of the claims 1 to
 27. 30. An intermediate productmanufactured after step (f), of the method as claimed in any one ofclaims 1 to
 27. 31. A method of producing a biodegradable foamedproduct, as claimed in any one of claims 1 to 27, substantially ashereinbefore described and with reference to the accompanying drawingand examples.
 32. A biodegradable foamed product, as claimed in claim28, substantially as hereinbefore described and with reference to theaccompanying drawing and examples.
 33. An intermediate product, asclaimed in either claim 29 or claim 30, substantially as hereinbeforedescribed and with reference to the accompanying drawing and examples.